Tumors are dependent upon new blood vessel formation, or angiogenesis, for expansive growth. Our recent analysis of gene expression in tumor endothelium led to the identification of a G-protein coupled receptor (GPCR) called Tumor endothelial marker 5 (TEM5). In an attempt to understand the role of TEM5 in angiogenesis, we have embarked upon a hunt for its physiological ligand. For this, we have generated stable TEM5 transfectants of 293 cells, and as a screen for ligand activity are taking advantage of the ability of most GPCRs to stimulate the release of calcium into the cytosol upon ligand stimulation. We will attempt to biochemically purify the TEM5 ligand from crude extracts of angiogenic tissues using the calcium mobilization assay as a screen. The cell based calcium assay may also be used to screen drug libraries to find natural or synthetic agonists or antagonitst of TEM5.